This invention relates to an exhaust gas purification device for an internal combustion engine and, more particularly, to the device for purifying nitrogen oxide (NOx) generated during a lean burn engine operation. The invention also relates to an air-fuel (A/F) ratio control for early activating a NOx trapping catalyst used in the exhaust gas purification device.
Recently, lean burn engines operable with a lean A/F ratio higher than a stoichiometric A/F ratio have been regarded effective from viewpoints of improvement in fuel economy and reduction of carbon dioxide (CO2) present in exhaust gas emitted from the engines. Exhaust generated from the lean burn engines has a high content of oxygen, whereby a generally used three-way catalyst cannot sufficiently reduce NOx present in the exhaust. There is a demand to provide a technique for effectively removing the NOx during the lean A/F ratio operation of the engine.
U.S. Pat. No. 5,473,887 discloses an exhaust gas purification device using a NOx trapping catalyst that is operative to trap NOx present in exhaust gas flowing into the NOx trapping catalyst when an A/F ratio of the exhaust gas is lean and reduce the NOx present in the exhaust gas when the A/F ratio of the exhaust gas becomes rich. In this device, the NOx trapping catalyst traps the NOx present in the exhaust gas emitted from the engine during the lean A/F ratio operation and the NOx trapped by the NOx trapping catalyst is reduced by temporarily operating the engine with the rich A/F ratio at a predetermined timing.
Meanwhile, when the engine is under the fully warm-up condition, the three-way catalyst and the NOx trapping catalyst are sufficiently heated to effectively purify the exhaust gas. When the engine is not under the fully warm-up condition, for instance, immediately after the starting up, the three-way catalyst and the NOx trapping catalyst do not reach the sufficiently heated state. This leads to considerable difficulty in purifying the exhaust gas. There have been proposed various techniques for activating the catalysts for a shorter period of time. One of the techniques is to arrange the catalyst in the exhaust system near an exhaust port of the engine. Namely, if the catalyst is located further upstream in the exhaust system, the heat of the exhaust gas emitted from the engine can be introduced into the catalyst without being transmitted to other parts of the exhaust system so that the catalyst can be activated for the shorter period of time.
Thus, in order to early commence the reduction of the NOx by the NOx trapping catalyst in the lean burn engine, it is desirable to arrange the NOx trapping catalyst near the exhaust port of the engine. On the other hand, generally, the NOx trapping catalyst has a heat resistance lower than that of the three-way catalyst. In some cases, the NOx trapping catalyst has a lower operating temperature range in which the trapping and reduction of the NOx can be achieved with high efficiency, as compared with a temperature of the exhaust gas near the exhaust port of the engine. Considering these characteristics of the NOx trapping catalyst, it will be undesirable to locate the NOx trapping catalyst near the exhaust port of the engine.
U.S. Pat. No. 5,388,403 discloses such an arrangement that the three-way catalyst is located upstream of the exhaust system and the NOx trapping catalyst is arranged downstream of the three-way catalyst. In this arrangement, the A/F ratio in a combustion chamber of an engine cylinder is made rich for providing stable combustion upon the staring up of the engine and the A/F ratio of the exhaust gas is made lean by introducing a secondary air from the upstream side of the three-way catalyst to promote the activation of the catalysts. The upstream three-way catalyst oxidizes hydrocarbon (HC) and carbon oxide (CO) present in the exhaust gas and the downstream NOx trapping catalyst traps the NOx present in the exhaust gas. When an amount of the NOx trapped by the NOx trapping catalyst reaches a predetermined value, the A/F ratio of the exhaust gas is made rich so as to reduce the NOx trapped in the NOx trapping catalyst.
In the conventionally proposed arrangement including the upstream three-way catalyst and the downstream NOx trapping catalyst, the exhaust gas having the A/F ratio made rich for the reduction of the NOx trapped by the NOx trapping catalyst is first introduced into the three-way catalyst. The HC and CO present in the exhaust gas react with the oxygen trapped by the three-way catalyst. The A/F ratio of the exhaust gas does not reach the rich ratio at an outlet port of the three-way catalyst until a whole amount of the oxygen in the three-way catalyst is consumed for the reaction with the HC and CO in the exhaust gas. Namely, the NOx present in the exhaust gas flowing into the NOx trapping catalyst cannot be reduced by the NOx trapping catalyst until the reaction of the whole amount of the oxygen is completed. It, therefore, will be required to supply the exhaust system with the exhaust gas with a fully rich A/F ratio that contains a sufficient amount of the reducing agent, i.e., HC and CO, to be used in the reaction with the oxygen in the three-way catalyst. This leads to decrease in fuel economy.
The three-way catalyst acts, because of its oxygen trapping property, to trap the oxygen present in the exhaust gas flowing thereinto when the exhaust gas is lean, and to oxidize the HC and CO present in the exhaust gas flowing thereinto by the trapped oxygen when the exhaust gas is rich. In order to lower the oxygen trapping property of the three-way catalyst for making the A/F ratio of the exhaust gas flowing into the NOx trapping catalyst rich, socalled ceria, i.e., cerium oxide (CeO2), generally carried by the three-way catalyst may be removed therefrom. However, if the oxygen trapping property of the three-way catalyst is lowered, then it will be undesirably caused to decrease the oxidation rate of the HC and CO present in the exhaust gas with the rich A/F ratio. Further, the oxygen trapping property of the three-way catalyst cannot be lowered to zero even if the ceria is removed from the three-way catalyst. This is because catalyst components other than the ceria can trap a certain amount of the oxygen present in the exhaust gas.
It is an object of the present invention to provide an exhaust gas purification device having catalysts parallel located and respectively connected to engine cylinder groups, and a NOx trapping catalyst located downstream of the catalysts, wherein the NOx present in the exhaust gas flowing into the NOx trapping catalyst can be reduced by making the exhaust gas flowing into one of the catalysts rich.
According to one aspect of the present invention, there is provided an exhaust gas purification device for an engine, comprising:
a first front catalyst disposed in a first exhaust passage connected to a first cylinder group, said first front catalyst being adapted to trap oxygen present in exhaust gas flowing thereinto when an air-fuel ratio of the exhaust gas is lean relative to a stoichiometric air-fuel ratio and oxidize a reducing agent present in the exhaust gas by the oxygen trapped thereby when the air-fuel ratio of the exhaust gas is rich relative to the stoichiometric air-fuel ratio;
a second front catalyst disposed in a second exhaust passage connected to a second cylinder group, said second front catalyst being adapted to trap oxygen present in exhaust gas flowing thereinto when an air-fuel ratio of the exhaust gas is lean relative to the stoichiometric air-fuel ratio and oxidize the reducing agent present in the exhaust gas by the oxygen trapped thereby when the air-fuel ratio of the exhaust gas is rich relative to the stoichiometric air-fuel ratio;
a rear catalyst disposed in a rear exhaust passage which combines the exhaust of the first and second exhaust passages, said rear catalyst being adapted to trap nitrogen oxides (NOx) present in exhaust gas flowing thereinto when an air-fuel ratio of the exhaust gas is lean relative to the stoichiometric air-fuel ratio and reduce the NOx trapped thereby by the reducing agent present in the exhaust gas when the air-fuel ratio of the exhaust gas is rich relative to the stoichiometric air-fuel ratio; and
a controller programmed to make the air-fuel ratio of the exhaust gas flowing into the first front catalyst rich relative to the stoichiometric air-fuel ratio and make the air-fuel ratio of the exhaust gas flowing into the second front catalyst stoichiometric or lean relative to the stoichiometric air-fuel ratio when the NOx trapped by the rear catalyst is reduced.
According to a further aspect of the present invention, there is provided an engine with an exhaust gas purification device, comprising:
a plurality of engine cylinders divided into a first cylinder group and a second cylinder group;
a first exhaust passage connected to the first cylinder group;
a second exhaust passage connected to the second cylinder group;
a common exhaust passage merging the first and second exhaust passages and disposed downstream thereof;
a first catalyst disposed in the first exhaust passage;
a second catalyst disposed in the second exhaust passage;
a nitrogen oxide (NOx) trapping catalyst disposed in the common exhaust passage;
fuel injectors connected to the engine cylinders; and
a controller connected to the fuel injectors and programmed to control fuel injection for making an air-fuel ratio of exhaust gas flowing into the first catalyst rich relative to a stoichiometric air-fuel ratio.
According to a still further aspect of the present invention, there is provided a method for controlling an air-fuel ratio of exhaust gas generated from engine cylinders and flowing into catalysts arranged in parallel upstream of a nitrogen oxide (NOx) trapping catalyst, said catalysts being operative to trap oxygen present in the exhaust gas flowing thereinto and oxidize a reducing agent present in the exhaust gas by the trapped oxygen, said method comprising:
calculating a basic injection fuel amount required to be injected into each engine cylinder for making an air-fuel ratio of an air-fuel mixture to be combusted a stoichiometric air-fuel ratio;
determining an optimal ignition timing based on the basic injection fuel amount calculated; and
calculating an additional injection fuel amount required to be injected into a predetermined engine cylinder at a predetermined period of time for making an air-fuel ratio of the exhaust gas generated from the predetermined engine cylinder and flowing into the NOx trapping catalyst rich.